PROJECT SUMMARY Small Cell Lung Carcinoma (SCLC) is an aggressive neuroendocrine subtype of lung cancer. SCLC patients have a very low 5-year survival, in part because SCLC tumors are often detected at a late stage when the tumors have already metastasized and treatment outcomes are worse. Thus, early detection becomes critical to achieve better treatment results. Emerging evidence supports the idea that, while SCLC tumors seem homogeneous when examined under a microscope, these tumors contain a significant level of intra-tumoral heterogeneity. Indeed, recent observations by our group and others have identified distinct cellular phenotypes in SCLC, including in primary human tumors, in cell lines derived from human tumors, and in tumors from genetically-engineered mouse models. Importantly, data from our group indicate that these cellular phenotypes contribute to SCLC development. The specific goal of this proposal is to elucidate how different cellular subpopulations within SCLC tumors drive early SCLC development, dynamics, and growth and to leverage this mechanistic information to identify biomarkers for early detection and prevention of SCLC. We have previously identified tumor-propagating cells (TPCs) in SCLC tumors and found that these cells are neuroendocrine and strongly tumorigenic. We have also characterized cell populations derived from these TPCs with distinct phenotypes, including non-neuroendocrine NOTCH+ and CD44+ subpopulations, that promote the growth and survival of the neuroendocrine TPCs. Leveraging these findings as well as our unique genetic mouse models that allow dissection of SCLC phenotype evolution, we will investigate how cell-cell interactions of these distinct SCLC cell phenotypes contribute to tumor development and growth, in relationship with the tumor microenvironment. We will also elucidate the role of secretory factors released by these SCLC subpopulations in driving survival, growth, and phenotype composition of SCLC tumors. Finally, we will perform analysis of cfDNA and proteins (including on exosomes) present in SCLC patient plasma for identification of related markers of SCLC development and early detection. We will also follow up on intriguing findings that germline mutations in NOTCH are present in a large fraction of SCLC patients, suggesting a potential risk marker beyond smoking. This interdisciplinary basic-translational project will elucidate fundamental mechanisms of SCLC development and may lead to novel methods for early detection and/or prevention of SCLC.